Various devices have been used to measure light for an information support. For example, U.S. Pat. No. 3,761,726 discloses a photoelectric device in which mirrors are used to focus light from an information support to a sensor. European Application 0 234 579 describes a reflection density measuring system used to monitor chemical assay slides.
In U.S. Pat. No. 4,639,607, a paper edge detector is described in which a beam of light is directed onto a small portion of a reflecting surface. The specular-reflected light from this surface then falls on the detector via a converging lens. Within certain limits the amount of light detected is independent of changes in orientation of the reflecting surface, although the distance between the exposed portion and the lens should stay constant.
A device of this kind can be used, for example, to determine the necessary exposure for background-free copying in an electrophotographic apparatus. To this end, the amount of light scattered by an information support is measured by exposing a portion of this support and measuring the amount of light scattered from the exposed portion. A disadvantage of this is that if there is any change in the distance between the information support and the sensor, the amount of light on the sensor may change. This occurs, for example, if the information support moves in a path along the sensor with possible variation of the positioning of the information support with respect to the sensor. Accordingly, it is an object of this invention to substantially reduce the disadvantage.